Such apparatus as described above are known commercially. The flat, smooth, pore-free surface of the glass or glass ceramic cooking top allows for easy cleaning. The cooking, grilling, or warming zones according to the prior art are heated by radiant heating elements having spirally wound resistance wires, or tubular heating elements, heating films, or halogen lamps. A temperature limiter is provided to protect the glass or glass ceramic cooking top from overheating. A cutoff temperature of 560° C. to 600° C. is selected for cooking operations. For a warming zone, a cutoff temperature of approximately 100° C. to 150° C. is selected.
Temperature limiters are expensive, with costs in the same range as the heating unit. To avoid temperature limiters, the specific heating capacity of the heating unit could be designed low enough so that overheating of the glass or glass ceramic cooking top would be prevented under all operating conditions. However, this would have the disadvantage of a very lengthy heat-up time, as well as considerable thermal inertia in supplying heat to a cold cooking utensil.
U.S. Pat. No. 5,220,155 describes a heating unit for a glass ceramic cooking top in which heating elements exhibiting PTC behavior are provided on a heat sink disk. The heat sink disk is made of an electrically insulating material with good thermal conductivity, such as aluminum nitrite. A disk made of such a material is costly.